This invention relates to a back projection type screen in which the adverse effect of the reflection of image light by the light incidence side surface of the screen is eliminated and a back projection type image display apparatus using the same.
In a back projection type image display apparatus according to the prior art, as schematically shown in FIG. 1 of the accompanying drawings, the image light from a display device 2 such as a CRT is expanded and projected onto a screen 6 through a projection lens 3. The screen 6 is usually comprised of a combination of a Fresnel lens sheet 6a as shown in FIG. 3 of the accompanying drawings and a lenticular lens sheet 6b. The Fresnel lens is formed by making a plano-convex lens shown at the bottom of FIG. 3 into a thin sheet, and carving variations in the curvature of the lens concentrically flatly.
The construction of FIG. 1 is actually housed in a cabinet 1 as shown in FIG. 2 of the accompanying drawings, and the image light has its optical path bent by reflecting mirrors 4 and 5 and is projected onto the screen 6. Heretofore, acrylic resin or the like has been used for the screen 6 and generally, the Fresnel lens sheet 6a whose incidence side surface is a non-lens surface, i.e., a planar surface, is disposed on the light incidence side, and the lenticular lens sheet 6b with a diffusing agent mixed therewith is disposed on the emergence side.
However, in the example of the prior art as described above there is the phenomenon that a small percentage of image light reflection occurs on the incidence side non-lens surface or the Fresnel lens surface of the Fresnel lens sheet 6a and this reflected light impinges on the reflecting mirrors 4 and 5 and returns to the screen 6 again.
Thus, the reflected light in the particularly intense portion of the light becomes intense, and when it is re-reflected to the dark portion of the screen, band-like or round images of R (red), G (green) and B (blue) appear (in the case of the three R, G and B tubes type) or the re-reflected light becomes flared and the quality of image is remarkably spoiled. Particularly, in the construction of an image display apparatus of the oblique incidence type for making the depth of the cabinet 1 small (the type in which an image light is projected onto a screen at an angle), this phenomenon is difficult to neglect.
Among the conventional screens, there is known one in which a plurality of cylindrical lenses are arranged on the light beam incidence surface of the sheet next to the light beam incidence side in a direction perpendicular to the generating line to thereby form a lenticular lens surface. In this case, this lenticular lens surface diffuses reflected light in one direction, but the reflected light is not diffused in a direction perpendicular to the direction of diffusion and therefore, if this reflected light is incident, for example, on the reflecting mirrors 4 and 5 and is again incident on the screen 6, it is diffused and its intensity weakens, but this also has been a cause for spoiling the quality of image as ghost or flare light extending in the direction of diffusion.